The invention relates to a method for the removal of particulates and for the simultaneous removal of SO.sub.2 and NO.sub.x from exhaust or combustion gases.
Flue gas resulting from the combustion of carbonaceous material can contain hundreds to thousands parts per million (PPM) SO.sub.2 and several hundred PPM of NO.sub.x. The NO.sub.x generallyis in the form of NO.
In previous flue gas clean-up processes, sulfur dioxide was removed by wet-scrubbing. In one method, the flue gas was passed through an aqueous slurry of lime or limestone for reaction with SO.sub.2 to form insoluble calcium sulfate and/or calcium sulfite. In a variation, known as the dual alkaline method, the flue gas passed through a scrubbing solution containing a sulfur dioxide absorbent such as sodium carbonate which reacted with SO.sub.2 to form sodium bisulfite. The scrubber solution was regenerated by contact with a lime or limestone slurry where the sodium bisulfite reacted with calcium to form large quantities of insoluble hydrated calcium sulfite and calcium sulfate. The insoluble salts typically were sent to landfills for disposal. These processes used alone were not effective in removing oxides of nitrogen, particularly NO, from exhaust gases.
In one effort to simultaneously remove oxides of sulfur and oxides of nitrogen (SO.sub.x and NO.sub.x) from exhaust gases, the gas was contacted with an aqueous absorbent containing a metal sulfite, ferrous ions and an aminopolycarboxylic acid. The aminopolycarboxylic acid removed the oxides of nitrogen and the oxides of sulfur were removed as dithionates (M.sub.2 S.sub.2 O.sub.6) in a crystallization separation. Unfortunately, the dithionates are difficult and expensive to decompose.
In an earlier effort the present inventor found that the liquid absorbent for the oxides of sulfur could be regenerated in an electrodialysis process and the resulting sulfur-containing material removed as sulfuric acid (U.S. Pat. No. 4,615,780). Although this process is effective in the simultaneous removal of SO.sub.2 and NO.sub.x from exhaust or combustion gases, various problems have arisen. The anion selective membranes used in the electrodialysis stack have been degraded by deposits of iron ethylene diamine tetraacetic acid (EDTA). The use of bipolar membranes for acid production prevented current reversal to remove the deposits. Also, the presence of acid exacerbated iron EDTA deposits and decreased current efficiency by hydronium ion permeation through anion selective membranes.
Therefore in view of the above, it is an object of the present invention to provide an improved process for the removal of oxides of sulfur and oxides of nitrogen from exhaust gases.
It is a further object to provide a process in which particular material can be removed along with the oxides of sulfur and oxides of nitrogen.
It is also an object to provide a regeneration method for an absorbent of SO.sub.2 in which electrodialysis is conducted at near neutral pH values.
It is one other object of the present invention to provide an electrodialysis regeneration method with reduced membrane fouling in the electrodialysis compartments.
It is yet a further object to provide an improved electrodialysis regeneration process compatible with the use of metal chelates for the absorption of NO.sub.x gases.